Back pain can be caused by a variety of factors including but not limited to the rupture or degeneration of one or more intervertebral discs due to degenerative disc disease, spondylolisthesis, deformative disorders, trauma, tumors and the like. In such cases, pain typically results from compression or irritation of spinal nerve roots arising from reduced spacing between adjacent vertebrae, a damaged disc and or misalignment of the spine resulting from the injury or degeneration.
Common forms of treating such pain include various types of surgical procedures in which a damaged disc may be partially or totally excised. After the disc space is prepared, one or more implants are inserted between the adjacent vertebrae in an effort to restore the natural spacing and alignment between the vertebrae, so as to relieve the compression, irritation or pressure on the spinal nerve or nerves and, thereby, eliminate or significantly reduce the pain that the patient is experiencing. Typically, one or more implants are used together with substances that encourage bone ingrowth to facilitate fusion between adjacent vertebrae and achieve immobilization of adjacent bones. Surgeons insert these intervertebral devices to adjunctively facilitate bone fusion in between and into the contiguous involved vertebrae. This fusion creates a new solid bone mass and provides weight bearing support between adjacent vertebral bodies which acts to hold the spinal segment at an appropriate biomechanically restored height as well as to stop motion in a segment of the spine and alleviate pain.
In a posterior lumbar interbody fusion (PLIF) surgery, spinal fusion is achieved in the lower back by inserting an implant such as a cage and typically graft material to encourage bone ingrowth directly into the disc space between adjacent vertebrae. The surgical approach for PLIF is from the back of the patient, posterior to the spinal column. An anterior lumbar interbody fusion (ALIF) surgical procedure is similar to the PLIF procedure except that in the ALIF procedure, the disc space is fused by approaching the spine through the abdomen from an anterior approach instead of from a posterior approach. Another fusion procedure is called a transforaminal lumbar interbody fusion (TLIF) which involves a posterior and lateral approach to the disc space. To gain access to the disc space, the facet joint may be removed whereby access is gained via the nerve foramen. In an extreme lateral interbody fusion (XLIF), the disc space is accessed from small incisions on the patient's side.
In the typical procedures described above, the adjacent vertebrae must be distracted apart by a substantial amount in order to allow the surgeon to advance the implant with relatively little resistance along the delivery path. Also, the surgeon must typically release the implant at least once as the implant is being delivered along the delivery path and align and position the implant at the target position of implantation, typically in the anterior aspect of the disc space. Once positioned, the interbody spacer is secured to the adjacent vertebrae with one or more bone screws. The implant includes apertures formed at one end for passing one or more bone screws at an upward angle into the first adjacent vertebral body and one or more bone screws at a downward angle into the second adjacent vertebral body.
Over time, the interface between the screws and the bone may present some problems of stability. Due to the anatomical structure of the spine and the extreme anatomical forces that are brought to bear on the skeleton and transmitted to the vertebral bodies, the screws securing the interbody spacer to the spine may vibrate or toggle out of position. Also, the degeneration of vertebral bone quality may result in the screws loosening or becoming dislodged. As a result, bone screws securing the plate to the spine may move or back out of the vertebral body and implant. Loosened screws may result instability of the joint and lead to increased pain for the patient.
Therefore, there is a need to provide a new and improved interbody spacer that resists fasteners, such as bone screws, from backing out and also from being loosened with respect to the implant before migrating out. Furthermore, there is a need for the implant to withstand anatomical forces and be easily implanted. Also, the screw retaining mechanism must be easily activated by the surgeon. This invention, as described in the detailed description, sets forth an improved interbody spacer that meets these needs.